Silicon carbide-based, porous structural material being heat-resistant and super lightweight

ABSTRACT

The present invention provides a silicon carbide-based heat-resistant, ultra lightweight, porous structural material having the same shape as that of a spongy porous body and also provides a process for readily producing the material. In the process of the present invention, slurry containing silicon powder and a resin is applied to the framework of the spongy porous body by an impregnation method in such a manner that interconnected pores of the porous body are not plugged with the slurry; the resulting porous body is carbonized at a temperature of 900° C. to 1320° C. in vacuum or in an inert atmosphere; the resulting porous body is subjected to reactive sintering at a temperature of 1320° C. or more in vacuum or in an inert atmosphere, whereby silicon carbide having high wettability to molten silicon is produced and open pores due to a volume reduction reaction are formed in one step; and molten silicon is infiltrated into the resulting porous body at a temperature of 1300° C. to 1800° C. in vacuum or in an inert atmosphere, whereby the silicon carbide-based heat-resistant, ultra-lightweight, porous structural material is produced.

TECHNICAL FIELD

The present invention relates to silicon carbide-based heat-resistant,ultra-lightweight, porous structural materials having a sponge structurewith interconnected pores, the materials being produced by a two-stepreactive sintering process including a step of sintering silicon andcarbon and a step of infiltrating molten silicon into the sintered body,and also relates to processes for producing the materials. The presentinvention particularly relates to a heat-resistant, ultra-lightweight,porous structural material fit for various applications such ashigh-temperature filters, high-temperature structural members, heatinsulators, filters for molten metal, burner plates, heater members, andhigh-temperature sound absorbers and also relates to a process forproducing the material.

BACKGROUND ART

Silicon carbide ceramics are light in weight and excellent in heatresistance, abrasion resistance, corrosion resistance, and so on.Therefore, such ceramics have been recently used in various applicationssuch as high-temperature corrosion-resistant members, heater members,abrasion-resistant members, abrasives, and grindstones. Since theceramics are principally produced by a sintering process, they have notbeen in practical use as ultra-lightweight porous members having aporosity of 90% or more and a filter shape.

In recent years, the porous ceramics having heat resistance and ultralightweight have been investigated. For example, Bridgestone Corporationhas succeeded in producing a porous silicon carbide structure used forceramic foam filters for cast iron according to the following procedure:a sponge is impregnated with silicon carbide slurry, and an excess ofthe slurry is removed from the resulting sponge, which is dried and thenfired. According to a catalogue showing properties, the porous siliconcarbide structure has a porosity of 85% and an apparent specific gravityof 0.42.

In the above procedure, since the slurry containing silicon carbidepowder is used, some pores are plugged with the remaining slurryalthough an excess of the slurry is removed from the sponge. Therefore,the porosity is 85%, which is a small value, and the apparent specificgravity is 0.42, which is a large value. Furthermore, the pore size isabout 1-5 mm (the standard number of cells ranges from 13 per 25 mm tosix per mm), which is a large value.

On the other hand, the inventors have obtained the following finding inthe investigation of a fiber-reinforced silicon carbide compositematerial: molten silicon hardly reacts with a dense matrix, prepared bythe carbonization of a phenol resin, containing only amorphous carbonbut readily permeate a porous matrix and reacts therewith, wherein theporous matrix contains residual amorphous carbon and silicon carbidethat is produced by the reactive sintering (volume reduction reaction)of a mixture of silicon powder and a phenol resin and has highwettability to the molten silicon, as disclosed in Japanese Patent No.3096716. Furthermore, the inventors have found that this technique canbe used for producing an ultra-lightweight, porous structural material.

DISCLOSURE OF INVENTION

In order to overcome disadvantages of known silicon carbide-basedheat-resistant, lightweight, porous materials and processes forproducing the materials, the present invention has been made based onthe above findings. The present invention provides a siliconcarbide-based heat-resistant, ultra lightweight, porous structuralmaterial and a process for producing the material, wherein the materialhas uniform pores therein, a porosity of 80% or more, and a density of0.3 g/cm³ or less. The material can be readily produced in such a mannerthat the shape of the framework of a porous body is maintained even ifthe shape is complicated.

As a result of an intensive investigation on the silicon carbide-basedheat-resistant, ultra-lightweight, porous structural material, theinventors have found that the material can be readily produced in such amanner that the shape of the framework of the porous body is maintainedeven if the shape is complicated according to the following procedure:silicon powder and a resin are allowed to adhere to the framework of theporous body such as a sponge by an impregnation method, porous siliconcarbide and residual carbon are produced from the silicon powder andresin by the silicon carbide production reaction in which volumereduction occurs, and molten silicon is then infiltrated into the pores.The present invention has thereby been completed.

The silicon carbide-based heat-resistant, ultra-lightweight, porousstructural material of the present invention completed as describedabove contains silicon carbide having high wettability to molten siliconand silicon provided in a carbonized porous sintered body, having openpores formed due to a volume reduction reaction, by melt infiltration.The carbonized porous sintered body is formed by the reactive sinteringof a carbonized porous body formed by carbonizing a spongy porous bodymade of plastic or paper for forming a framework, the porous body beingimpregnated with slurry containing silicon powder and a resinfunctioning as a carbon source in such a manner that interconnectedpores of the porous body are not plugged with the slurry.

A process for producing the silicon carbide-based heat-resistant,ultra-lightweight, porous structural material of the present inventionincludes a step of applying slurry, containing silicon powder and aresin functioning as a carbon source, to the framework of a spongyporous body made of plastic or paper by an impregnation method in such amanner that interconnected pores of the porous body are not plugged withthe slurry; a step of carbonizing the resulting porous body at atemperature of 900° C. to 1320° C. in vacuum or in an inert atmosphere;a step of subjecting the resulting porous body to reactive sintering ata temperature of 1320° C. or more in vacuum or in an inert atmosphere,whereby silicon carbide having high wettability to molten silicon isproduced and open pores due to a volume reduction reaction are formed inone step; and a step of infiltrating molten silicon into the resultingporous body at a temperature of 1300° C. to 1800° C. in vacuum or in aninert atmosphere.

In the above process, the reactive sintering of silicon and carbon andthe melt infiltration of silicon may be performed in the sameheat-treating step and all heat-treating operations including thecarbonization may be performed in the same step.

According to the process of the present invention, large-sizedstructures with a complicated shape can be readily produced and porousbodies can be readily machined after the carbonization thereof.

In the above process, in order to impregnate the porous body with theslurry in such a manner that the interconnected pores are not pluggedwith the slurry, the following procedure is effective: the slurrycontaining the resin and silicon powder is applied to the framework ofthe porous body by an impregnation method and the slurry is then wrungout of the resulting porous body. Examples of a method for wring theslurry include a compression method and a method using the centrifugalforce.

In the above process, a material for forming the framework of the spongyporous body preferably retains the slurry and examples of such amaterial include a sponge containing a resin or rubber, spongy plastic,and spongy paper.

In the above process, examples of the resin allowed to adhere to theframework of the porous body by an impregnation method include a phenolresin, a furan resin, an organic metal polymer such as polycarbosilane,and sucrose. These materials may be used alone or in combination.Examples of the additive include carbon powder, graphite powder, andcarbon black. Examples of the aggregate or oxidation inhibitor includesilicon carbide, silicon nitride, zirconia, zirconium, alumina, silica,mullite, molybdenum silicide, boron carbide, and boron powder.

In the above process, the silicon powder contained in the slurry maycontain a silicon alloy containing at least one selected from the groupconsisting of magnesium, aluminum, titanium, chromium, manganese, iron,cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, andtungsten or the slurry may contain a mixture of the silicon powder andthose metals. Furthermore, silicon for melt infiltration may be a puresilicon metal or may be derived from a silicon alloy containing oneselected from the group consisting of magnesium, aluminum, titanium,chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium,niobium, molybdenum, and tungsten or derived from a mixture of siliconand those metals.

According to the silicon carbide-based heat-resistant,ultra-lightweight, porous structural material and production process ofthe present invention, the slurry containing the silicon powder and theresin functioning as a carbon source is applied to the framework of thespongy porous body by an impregnation method in such a manner that theinterconnected pores of the porous body are not plugged with the slurry,silicon carbide having high wettability to molten silicon and the openpores are formed by the reactive sintering, and molten silicon is theninfiltrated into the pores. Therefore, a silicon carbide-basedheat-resistant, lightweight, porous composite material having the sameshape as that of the porous structural material can be readily produced.Thus, the porous composite material can be readily produced even if ithas a complicated shape.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described.

In a process of the present invention, slurry is prepared by mixingsilicon powder with a dissolved resin, such as a phenol resin,functioning as a carbon source; the slurry is sufficiently applied ontothe framework of a spongy, porous structural material or the porousstructural material is immersed in the slurry such that the porousstructural material is impregnated with the slurry; the slurry is wrungout of the resulting porous structural material in such a manner thatinterconnected pores of the porous structural material are not pluggedwith the slurry; and the resulting porous structural material is thendried. The porous structural material is preferably dried at about 70°C. for about 12 hours.

Examples of the porous structural material include sponges containing aresin or rubber, spongy plastics, and spongy paper.

The resin allowed to adhere to the framework of the porous structuralmaterial is at least one selected from the group consisting of a phenolresin, a furan resin, an organic metal polymer, and sucrose. The resinmay contain the above additive and the like according to needs.

The silicon powder for forming silicon carbide preferably has a fineparticle size. In particular, the average particle size is preferably 30μm or less. The silicon powder having a large particle size may bepulverized in a ball mill.

Next, the resulting porous structural material is carbonized at atemperature of 900-1320° C. in vacuum or in an inert atmosphere such asan argon atmosphere. In this operation, the spongy porous structuralmaterial is thermally decomposed, whereby a carbonized compositematerial is obtained. The framework of the carbonized composite materialcontains carbon produced by the carbonization of the phenol resin andthe silicon powder, the carbon and silicon powder being mixed together.The shape of the carbonized composite material is the same as that ofthe porous structural material. The carbonized porous structuralmaterial has a strength sufficient for machining.

The carbonized porous structural material is fired at a temperature of1320° C. or more in vacuum or in an inert atmosphere such as an argonatmosphere such that carbon reacts with silicon, whereby porous siliconcarbide having high wettability to molten silicon is formed on theframework of the material. Since the volume is reduced in this reaction,open pores are formed due to the volume reduction reaction. This resultsin a porous sintered body having a matrix portion containing poroussilicon carbide and residual carbon.

The porous sintered body is heated to a temperature of 1300-1800° C. invacuum or in an inert atmosphere, and molten silicon is infiltrated intoporous portions of the framework containing silicon carbide and carbon,whereby a silicon carbide-based heat-resistant, ultra-lightweight,porous structural material is obtained.

According to the present invention, in the mixture of the silicon powderand carbon derived from the resin, the molar ratio of silicon to carbonis preferably within a range of 0.05 to 4.

EXAMPLES

A process of the present invention will now be described in detail withreference to examples. The present invention is not limited to theexamples.

Example 1

The mixing ratio of a phenol resin to silicon powder was set such thatthe molar ratio of carbon formed by the carbonization of the phenolresin to silicon is five to three. The phenol resin was dissolved inethyl alcohol, thereby preparing slurry. In order to reduce the size ofthe silicon particles, the slurry was mixed in a ball mill for one day.The slurry was infiltrated into a polyurethane sponge having pores witha size of 500-600 μm. The resulting sponge was wrung in such a mannerthat the interconnected pores are not plugged with the slurry. Theresulting sponge was then dried. In this operation, the sponge wasexpanded in the axial direction by about 20%.

The resulting sponge was fired at 1000° C. for one hour in an argonatmosphere, thereby carbonizing the sponge. The obtained carbonaceousporous body was heated at 1450° C. for one hour in vacuum, therebyperforming reactive sintering and the melt infiltration of silicon inone step. A silicon carbide-based heat-resistant, ultra-lightweight,porous structural material having the same shape as that of the spongewas then obtained. In the carbonizing operation, the sponge was slightlyreduced in size because the carbonized sponge shrunk in the axialdirection by about 12% as compared with the untreated sponge.

The obtained porous structural material had the same structure as thatof the sponge and also had a pore diameter of 500-600 μm, a porosity of97%, and a density of 0.07 g/cm³. The porous structural material did nothave plugged pores.

Example 2

The mixing ratio of a phenol resin to silicon powder was set such thatthe molar ratio of carbon formed by the carbonization of the phenolresin to silicon is five to three. The phenol resin was dissolved inethyl alcohol, thereby preparing slurry. In order to reduce the size ofthe silicon particles, the slurry was mixed in a ball mill for one day.The slurry was infiltrated into a polyurethane sponge having pores witha size of about one mm. The resulting sponge was wrung in such a mannerthat the interconnected pores are not plugged with the slurry. Theresulting sponge was then dried. In this operation, the sponge wasexpanded in the axial direction by about 20%.

The resulting sponge was fired at 1000° C. for one hour in an argonatmosphere, thereby carbonizing the sponge. The obtained carbonaceousporous body was heated at 1450° C. for one hour in vacuum, therebyperforming reactive sintering and the melt infiltration of silicon inone step. A silicon carbide-based heat-resistant, ultra-lightweight,porous structural material having the same shape as that of the spongewas then obtained. In the carbonizing operation, the sponge was slightlyreduced in size because the carbonized sponge shrunk in the axialdirection by about 12% as compared with the untreated sponge.

The obtained porous structural material had the same structure as thatof the sponge and also had a pore diameter of about one mm, a porosityof 97%, and a density of 0.06 g/cm³.

Example 3

The mixing ratio of a phenol resin to silicon powder was set such thatthe molar ratio of carbon formed by the carbonization of the phenolresin to silicon is five to three. The phenol resin was dissolved inethyl alcohol, thereby preparing slurry. In order to reduce the size ofthe silicon particles, the slurry was mixed in a ball mill for one day.The slurry was infiltrated into a polyurethane sponge having pores witha size of about 1.5-2 mm. The resulting sponge was wrung in such amanner that the interconnected pores are not plugged with the slurry.The resulting sponge was then dried. In this operation, the sponge washardly expanded.

The resulting sponge was fired at 1000° C. for one hour in an argonatmosphere, thereby carbonizing the sponge. The obtained carbonaceousporous body was heated at 1450° C. for one hour in vacuum, therebyperforming reactive sintering and the melt infiltration of silicon inone step. A silicon carbide-based heat-resistant, ultra-lightweight,porous structural material having the same shape as that of the spongewas then obtained. In the carbonizing operation, the sponge was slightlyreduced in size because the carbonized sponge shrunk in the axialdirection by about 12%.

The obtained porous structural material had the same structure as thatof the sponge and also had a pore diameter of about 1.5-2 mm, a porosityof 95%, and a density of 0.1 g/cm³.

Comparative Example 1

The same sponge as that used in Example 1 was fired at 1000° C. for onehour in an argon atmosphere. As a result, the sponge was vanished.Comparative Example 2

A phenol resin was dissolved in ethyl alcohol, thereby preparing slurry.The slurry was infiltrated into a polyurethane sponge having a porediameter of 500-600 μm. The resulting sponge was wrung in such a mannerthat the interconnected pores are not plugged with the slurry. Theresulting sponge was then dried.

The resulting sponge was fired at 1000° C. for one hour in an argonatmosphere, thereby carbonizing the sponge. The obtained carbonaceousporous body was heated at 1450° C. for one hour in vacuum, therebyperforming reactive sintering and the melt infiltration of silicon inone step. However, the infiltration of silicon did not occur. Therefore,the carbonaceous porous body remained as it was.

Industrial Applicability

As described above in detail, according to a silicon carbide-basedheat-resistant, ultra-lightweight, porous structural material of thepresent invention, a silicon carbide-based heat-resistant, lightweight,porous composite material having the same shape as that of the porousstructural material can be produced. Therefore, the porous compositematerial can be readily produced even if it has a complicated shape.Thus, the porous structural material can be used in various applicationssuch as high-temperature filters, high-temperature structural members,heat insulators, filters for molten metal, burner plates, heatermembers, and high-temperature sound absorbers.

1. A silicon carbide-based heat-resistant, ultra lightweight, porous structural material containing silicon carbide having high wettability to molten silicon and silicon provided in a carbonized porous sintered body, having open pores formed due to a volume reduction reaction, by melt infiltration, wherein the carbonized porous sintered body is formed by the reactive sintering of a carbonized porous body formed by carbonizing a porous body made of plastic or paper for forming a framework, the porous body being impregnated with slurry containing silicon powder and a resin functioning as a carbon source in such a manner that interconnected pores of the porous body are not plugged with the slurry.
 2. The porous composite material heat-resistant, ultra lightweight, porous structural material according to claim 1, wherein the resin, allowed to adhere to the framework by an impregnation method, functioning as a carbon source is at least one selected from the group consisting of a phenol resin, a furan resin, an organic metal polymer, and sucrose.
 3. The porous composite material heat-resistant, ultra lightweight, porous structural material according to claim 1, wherein the slurry applied to the framework by an impregnation method contains an additive selected from the group consisting of carbon powder, graphite powder, and carbon black.
 4. The porous composite material heat-resistant, ultra lightweight, porous structural material according to claim 1, wherein the slurry applied to the framework by an impregnation method contains an aggregate or oxidation inhibitor that is at least one selected from the group consisting of silicon carbide, silicon nitride, zirconia, zirconium, alumina, silica, mullite, molybdenum silicide, boron carbide, and boron powder.
 5. The porous composite material heat-resistant, ultra lightweight, porous structural material according to claim 1, wherein the silicon powder contained in the slurry contains a silicon alloy containing at least one selected from the group consisting of magnesium, aluminum, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, and tungsten or the slurry contains a mixture of the silicon powder and those metals.
 6. The porous composite material heat-resistant, ultra lightweight, porous structural material according to claim 1, wherein silicon for melt infiltration is derived from a silicon alloy containing at least one selected from the group consisting of magnesium, aluminum, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, and tungsten or derived from a mixture of silicon and those metals.
 7. A process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material comprising a step of applying slurry, containing silicon powder and a resin functioning as a carbon source, to the framework of a spongy porous body, made of plastic or paper, by an impregnation method in such a manner that interconnected pores of the porous body are not plugged with the slurry; a step of carbonizing the resulting porous body at a temperature of 900° C. to 1320° C. in vacuum or in an inert atmosphere; a step of subjecting the resulting porous body to reactive sintering at a temperature of 1320° C. or more in vacuum or in an inert atmosphere, whereby silicon carbide having high wettability to molten silicon is produced and open pores due to a volume reduction reaction are formed in one step; and a step of infiltrating molten silicon into the resulting porous body at a temperature of 1300° C. to .1800° C. in vacuum or in an inert atmosphere.
 8. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 7, further comprising a step of wring the slurry, applied to the framework, containing the silicon powder and the resin, out of the porous body such that the interconnected pores of the porous body are not plugged with the slurry.
 9. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 7, wherein the resin allowed to adhere to the framework of the porous body by an impregnation method is at least one selected from the group consisting of a phenol resin, a furan resin, an organic metal polymer, and sucrose.
 10. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 7, wherein the slurry applied to the framework of the porous body by an impregnation method contains an additive selected from the group consisting of carbon powder, graphite powder, and carbon black.
 11. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 7, wherein the slurry applied to the framework of the porous body by an impregnation method contains an aggregate or oxidation inhibitor that is at least one selected from the group consisting of silicon carbide, silicon nitride, zirconia, zirconium, alumina, silica, mullite, molybdenum silicide, boron carbide, and boron powder.
 12. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 7, wherein the silicon powder contained in the slurry contains a silicon alloy containing at least one selected from the group consisting of magnesium, aluminum, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, and tungsten or the slurry contains a mixture of the silicon powder and those metals.
 13. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 7, wherein the silicon for melt infiltration is derived from a silicon alloy containing at least one selected from the group consisting of magnesium, aluminum, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, and tungsten or derived from a mixture of silicon and those metals.
 14. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 8, wherein the resin allowed to adhere to the framework of the porous body by an impregnation method is at least one selected from the group consisting of a phenol resin, a furan resin, an organic metal polymer, and sucrose.
 15. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 8, wherein the slurry applied to the framework of the porous body by an impregnation method contains an additive selected from the group consisting of carbon powder, graphite powder, and carbon black.
 16. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 8, wherein the slurry applied to the framework of the porous body by an impregnation method contains an aggregate or oxidation inhibitor that is at least one selected from the group consisting of silicon carbide, silicon nitride, zirconia, zirconium, alumina, silica, mullite, molybdenum silicide, boron carbide, and boron powder.
 17. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 8, wherein the silicon powder contained in the slurry contains a silicon alloy containing at least one selected from the group consisting of magnesium, aluminum, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, and tungsten or the slurry contains a mixture of the silicon powder and those metals.
 18. The process for producing a silicon carbide-based heat resistant, ultra lightweight, porous structural material according to claim 8 wherein the silicon for melt infiltration is derived from a silicon alloy containing at least one selected from the group consisting of magnesium, aluminum, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, and tungsten or derived from a mixture of silicon and those metals. 